The present invention relates to an apparatus and method for exposing a treatment area in a passage inside a patient to x-ray radiation. In particular, the invention relates to a method and apparatus for x-ray treatment supplying voltage pulses to an x-ray emitter.
X-ray emitters for medical uses, components of the emitter, and various delivery systems for positioning such a catheter in a passage inside the body of a patient have been described in other co-pending patent applications, such as patent application Ser. No. 08/701,764, "X-RAY CATHETER", the contents of which are hereby incorporated by reference herein.
X-ray devices include a cathode, an anode, and a housing. An electrode applies a voltage to the cathode to cause electron emission. In many x-ray devices it is common to use a thermionic cathode, or a hot cathode, in which the emission properties depend on the temperature of the cathode surface. A hot cathode has an additional electrode providing a low voltage current for heating the cathode. By raising the temperature of the cathode, the emission properties of the cathode improve and the current at the anode improves.
In thermionic cathodes, the anode current and voltage can be controlled and stabilized independently from each other. For example, the voltage may be varied while the anode current, that is directly related to the power deposited at the anode, is kept constant. An x-ray catheter that can be operated so that the voltage and the anode current are controlled and stabilized independently is useful for medical applications.
In many x-ray devices, a field emission cathode, or a cold cathode, is preferred to a hot cathode. Space limitations in a very small device may eliminate the possibility of a third electrode. The desire to avoid heat generation at the x-ray emitter may also preclude the use of a hot cathode.
In a field emission diode, an independent control of the voltage and the current cannot be obtained as described above. The field emission cathode is powered by a direct current high voltage and the value of the field emission current is directly related to the applied voltage. The I-V characteristic of the diode, defined by the Fowler-Nordheim law, is a very steep exponential function of voltage. That is, with increasing voltage, the current increases exponentially. The electrical power developed at the anode is an even steeper function of voltage.
In some medical applications where the x-ray catheter is used, such as the treatment of cancer, it is necessary to operate the x-ray emitter under different applied voltages depending on the patient and the site to be treated. Generally speaking, x-ray radiation emitted at a higher operating voltage will penetrate deeper in the tissue than does x-ray radiation emitted at a lower operating voltage. Additionally, the radiation dose, proportional to the anode current, and the time of irradiation, must be individually selected for every treatment.
One way of overcoming this problem would be to provide a number of x-ray catheters having different configurations so that they operate at different voltages within the necessary range. To obtain the desired treatment, an emitter of the proper operating voltage would be selected, and the tissue would be irradiated until the desired dose had been delivered. However, this solution is impractical because it requires the production of a number of different x-ray emitters with different anode-cathode characteristics which is not desirable from a manufacturing and cost point of view.
Another issue that arises during use of field emission cathodes is incidental heat generation. If an emitter having a field emission cathode operates at a high current for a long time, its temperature can reach undesired levels.
A voltage source is needed for x-ray devices that provides flexibility in supplying voltage and in current requirements, and provides other advantages. A voltage source that minimizes heat production and other disadvantages of a field emission cathode would also be valuable.